The present invention relates to an electrophotographic copying machine and, more particularly, to a device for removing charges unnecessary for forming a toner image on a latent image bearing element in an electrophotographic copying machine.
An electrophotographic copying machine produces an electrostatic latent image on a latent image bearing element such as a light sensitive photoconductive element with an optical system based on a reflected light image. The light sensitive photoconductive element is uniformly charged. The latent image corresponds to an image on a copy document such as a manuscript or book to be copied. Toner particles are electrically adhered to the latent image, so that the latent image becomes visible to form a toner image. The toner image is transferred onto a copy paper via a transference charger and fixed thereon.
Conventionally, the light sensitive photoconductive element is uniformly charged with a corona charger over its width. Therefore, when, for example, a reduced copy from the document is projected onto the light sensitive element, some nonused charges will remain at the portions of the light sensitive element except the projected portions. The development of this charge image at the developing section results in the fact that the toner particles are adhered on these portions as well as at the latent image portions. By the transference charger, an unnecessary toner fixed image may be produced around the periphery of the copied paper. Since the toner particles are adhered to the portions unnecessary for forming the toner image, the amount of toner particles is quickly reduced.
To prevent the above disadvantage, a device is presented for removing the charges on the portions unnecessary for the toner image. Conventionally, such a charge removing device acts to emit light on the portions unnecessary for the image, so that the charges are discharged. This device is positioned between the charger unit and the developing unit.
FIG. 1 shows a plan view of a conventional charge removing device. In FIG. 1, a light shade plate 3 is positioned between a light sensitive element 1 and a light source 2. To remove the charges unnecessary for the toner image on the light sensitive element 1, in accordance with the positions of the unnecessary portions of the light sensitive element 1, some light sources 2 are illuminated or, otherwise, the light shade plate 3 is moved at the desired image portions a to shade these portions.
However, it may be difficult to position this device adjacent the light sensitive element because of the need of a large area of space for this device. In case this device is positioned far from the light sensitive element, as FIG. 1 shows, the light from the light source 2 radiates. The light from the light source 2-1 is shut out by the light shade plate 3, so that the unnecessary charges at c position in unnecessary b portions cannot be removed. The light from the light source 2--2 removes the charges at the d portions in the desired a portions for the image. Here, the boundary between the a and the b portions is vague and the necessary charges may be removed. Of course, the light shade plate 3 may be positioned adjacent the light sensitive element so as to make the boundary between the a and the b portions clear. However, because of the space limitation, such a positioning may be difficult in many cases as stated above.
Further, even if a sufficient space for positioning the light exposuring device adjacent the light sensitive element is present, a specific material for the light sensitive element such as a three-layered material is usually used for improving charge bearing feature of the light sensitive element. In such a case, since a simultaneous charge and light exposure process should be applied, it is necessary to interpose a charger between the light sensitive element and the charge removing device, whereby the charge removing device must be positioned far from the light sensitive element owing to the presence of the charger. It is difficult to make the boundary between the a and the b portions clear, as such.